SPECT systems are often used to show the distribution of a radioactive substance inside a patient's body. A source of penetrating radiation is administered to the patient, which typically consists of a pharmaceutical tagged with a radionuclide which emits radiation photons (radiopharmaceutical). The radiopharmaceutical is designed to be absorbed in a target organ, such as the heart muscle, or other organs or body part of interest. The emitted radiation photons are collimated with a collimator subsystem and detected by a detector subsystem which generates output electrical signals which are digitized and processed by a computer system to generate images of the regional distribution of the radioactive sources in and around the target organ.
TCT systems are typically used for obtaining images, or more precisely, the distribution (maps) of attenuation coefficient, in the human body. It has been generally accepted in the field of nuclear imaging that quantitative SPECT imaging requires proper attenuation correction, which in turn requires appropriate attenuation maps to facilitate its correction. The general way to derive these attenuation maps requires the use of TCT, which yields CT images that are converted to appropriate attenuation maps. As a result of this requirement, many modern SPECT systems come with a TCT system to allow TCT imaging to be performed either simultaneously with or sequential to the SPECT imaging. Such systems are referred to here as integrated SPECT/TCT systems, in contrast to recently introduced Hybrid SPECT/CT systems that combine a conventional SPECT with an independent x-ray-CT in the same granty.
One prior SPECT system of a typical SPECT/TCT system proposed by the inventor hereof utilizes a large circular shape design for the frame, or the gantry, collimator subsystem, and the detector subsystem which attempted to accommodate a large patient cross-section while placing the patient's heart at the geometric center for imaging from multiple directions simultaneously. However, because of the off-center location of heart, the circular geometry had to be fairly large to enclose a large patient's thorax. As a result, the long-distance collimation offsets the potential gain in geometric efficiency and renders the circular design less than optimum. Furthermore, the design devoted considerable collimator and detector area to the patient's right-posterior side, where the heart is too distant from the collimator for effective collimation. The typical problem of low photon sensitivity in SPECT is further compounded in cardiac imaging where the desirable radiation photons are scarce: only about 2-4% of the injected dose is in the myocardium of the heart. This previous circular design approach results in a limited return of the heavily attenuated and scattered photons and sub-optimal image quality.
The TCT system of a typical conventional SPECT/TCT system utilizes either a scanning line source of radiation, e.g., Gd155, or multiple lines sources stationary with respect to the gamma camera detector to create parallel photon-beams toward a patient to derive a transmission projection, simultaneously with a corresponding emission projection acquisition. These transmission projections, accumulated over a large number of directions, as the gamma camera rotates to meet the sampling requirements, are reconstructed to yield the attenuation maps. The drawback of this approach is that the photon statistics is quite poor because the finite flux of the relative narrow parallel-beam of radiation has to be spread over a large detector area.
Another type of TCT system of a SPECT/TCT system uses a stationary line source to provide a three-dimensional symmetrical or asymmetrical fan beam of photons, or a point source to provide a cone-beam, of radiation photons, towards a patient. A gamma camera located behind the patient includes a matched fan-beam or come-beam collimator for collimating the transmitted photons. The detector system detects the photons and outputs electrical signals and computer system generates TCT images based on the detected photons, after the camera and the source rotate around the patient in a sequential acquisition to the SPECT imaging. However, the problem with these SPECT/TCT systems is the required special collimation that either cannot be fully utilized in emission imaging of the heart, or the systems require a cumbersome and time-consuming collimator exchange procedure, which disturbs the imaging position of the patient and makes the subsequent matching of SPECT and TCT slices for attenuation correction difficult.
The collimator subsystems of typical conventional SPECT/TCT systems are designed with only one predefined set of collimation parameters for both SPECT and TCT imaging. However, such a design compromises either SPECT or TCT imaging. For different SPECT imaging requirements, or for patients having different sizes, a different set of collimation parameters is often needed. Therefore a different collimator with different collimation parameters is needed. Changing collimator, not only is awkward, cumbersome, time-consuming, and unrealistic in clinical environment, it is also difficult to reproduce patient imaging position after the collimator has been changed. The reproducibility of patient imaging position is critical when implementing attenuation correction and guiding image fusion.
The result is conventional SPECT/TCT systems are not flexible in accommodating different SPECT collimation requirements to suit various clinical situations and patients having different sizes and do not provide adequate TCT images needed for attention maps for high quality SPECT imaging. Such a limitation compromises either SPECT or TCT imaging, which in turn limits the attainable quality of attenuation-corrected SPECT imaging.
Another conventional SPECT/TCT system combines an independent x-ray CT, ranging from primitive low-end versions to expensive high-end multi-slice Spiral CTs, to the SPECT system, and are often referred to as Hybrid SPECT/CT systems. The concern of using this approach for cardiac SPECT/TCT is the cost, because an additional dedicated x-ray detector system is required, even for the primitive low-end CTs. As for the use of an expensive high-end multi-slice Spiral CT for attenuation of cardiac SPECT, it is still controversial and is not cost efficient. This latter approach is really intended for other clinical diagnostic applications, such as calcium scoring in cardiac vessels and/or detailed anatomic correlation for other imaging tasks in other regions of the body, rather than for attenuation correction of cardiac SPECT. In fact, due to the mismatch of the acquisition speeds of SPECT and high-end CT, e.g., 10 to 20 minutes for SPECT, verses about 1 to 2 seconds for TCT, the attenuation maps derived by high-end CT have to be further processed to include breathing motion before application. This processing is not trivial and is a potential source of artifacts and sub-optimal correction.
Additionally, conventional SPECT systems of prior SPECT/TCT systems typically incrementally rotate the large, heavy collimator and the detector subsystem about the patient to obtain a plurality of projection images (projections). Each time the collimator and the detector subsystem are rotated step-by-step, the collimator and detector follow the patient's body contour by successively adjusting their radial and lateral positions. Such a technique is cumbersome, not easily reproducible, prone to both mechanical and electrical errors, slow, inefficient, utilizes expensive hardware to rotate large heavy collimator and detector subsystems, and requires extensive safety measures to protect the patient. As a result, conventional SPECT images have large variations in image quality and reproducibility, which make comparison of images from different facilities or from different times at the same facility difficult.